Supersonic measuring means



Nov. 18, 1947.

W. S. ERWIN suPERsoNIC MEASURING MEANS Filed April 21, 1944 2Sheets-Sheet 1 Nov. 18, 1947. w. s. ERwlN 2,431,233

SUPERSONI C MEASURING MEANS Filed April 21, 1944 2 Sheets-Sheet 2INVENTCRS es@ f ATTORNEYS fore, would be one Patented Nov. 18, 19472,431.233 SUPERSONIC MEASURING MEANS Wesley S. Erwin, Detroit. Mich.,asslgnor to General Motors Corporation, poration of Delaware Detroit.Mich., a cor- Application April 21, 1944, Serial No. 532,199

11 claims.

'Ihis invention relates to means useful for measuring, gauging, sortingor structural study by the use of a variation in the electricalcharacteristics of a member. It relates more specifically to means usinga crystal which is mechanically coupled to a part being investigated.-the electrical characteristics of said crystal being utilized in thesystem to provide indications of the various conditions. A

There are, o1' course, many instances in which it is desirable todetermine the thickness of the walls forming a hollow member after thesame has been completely fabricated or manufactured, and it isimpossible to have access to the inside surface. As an example of thisit might be very desirous to ascertain the thickness of a wall of ahollow body after the same wall has been fixedly secured thereto andafter some machining has been done to finish the outside surface. Inmany instances these surfaces are ground away in order to provide theproper contour or smooth surface to too great an extent to maintain theproper strength of the material. A specific problem in this regard thatmight be cited is a modern propeller -blade which is fabricated byapplying to a concave casting or forging a sheet of metal which iswelded thereto around its edges and thereafter its outer surface isground and finished to speciflcations. As is well-known, the stressapplied to the propellers is very high and they must be carefullyinspected for thickness of this sheet when finished before they areapproved. This, thereinstance in which it is absolutely necessary tohave some means for gauging the thickness of this applied sheet in thefinished propeller.

It is also often necessary to provide means for measuring the bondbetween pieces which are secured together over substantial areas and inthe same category to be able to indicate or ascertain material soundnessor continuity of materials and by doing so classify or sort the membersbeing tested.

It is therefore an object of my invention to provide testing meansapplicable to materials in which only one surface is available.

It is a still further object ofl my invention to provide testing meansfor thin materials operating by the use of supersonic waves.

It is a still further object oi my invention to provide gauging meansoperated on the principle of setting up standing waves which resonate inthe body, thus determining the resonant wave length and the proportionalthicknesses.

It is a still further object of my invention to provide measuring meansoperating on the principle of measuring the variation in electricalcharacteristics of a transducer due to mechanical loading thereof.

It is a still further object of my invention to (Cl. Y73-67) providemeasuring means operated on the principleof measuring electrically themechanical loading on a piezoelectric crystal powered by an adjustablefrequency oscillator.

It is a still further object of my invention to provide measuring ortesting means which is small, portable and 'extremely easy to operate.

With these and other objects in view, which will become apparent as thespecification proceeds, the embodiments of my invention are bestunderstood by reference to the following specilication and claims andthe illustrations in the accompanying drawings, in which:

Figure l is a perspective view showing the apparatus of my invention.

Figure 2 is an enlarged sectional view taken through the meanssupporting the loaded part. in this instance a piezoelectric crystal.

Figure 3 is a schematic wiring diagram of the system incorporating myinvention, and

Figures 4 and 5 are wiring diagrams of a portion of the apparatusshowing modified forms of connecting the indicating meter into thecircuit.

Referring now more specifically to Figure 3 of the drawings, there isshown therein an electron oscillator tube indicated generally at 2,which has a plurality of electrodes therein, namely, a plate 4, asuppressor grid 6, a screen grid 8. a control grld I0. and a cathode I2.A main inductance I4 across which is connected a variable condenser I6forms the main tunable circuit for the input of the oscillator tube 2,and, as shown, is connected through line I8, condenser I9, and grid leak20 to the control grid I 0 of said tube. 'I'he inductance coll I4 isprovided with an adjustable tap 22 for cathode feedback. A trimmercondenser 24 ls also conne ted in parallel with the main tuningcondenser 6 to permit alignment of the two tuned circuits to the samefrequency.

One side of the main tuning condenser I6, one side of the trimmer 24,and one side of inductance coil I4 is grounded. Tap 22 of the same coilis connected through line 30 to the cathode I2 of tube 2. Grid 8 isconnected through line 32 to a resistor 34 and thence to a variable tap36 on resistance member 38, one terminal of which is grounded and theopposite terminal is connected through line 40 to a choke coil 42 andlter condenser 44 which in combination with a second filter condenser 46form a filter circuit between the resistor 38 and the voltage supply.Condenser 48 is connected between line 32 and the line 26.

Plate 4 of the tube 2 is connected through line 50 to a blockingcondenser 52 and thence through the shielded cable |26 by line 54 to thecrystal 58 which is seated on the work. The cable shield |26 is groundedas is the crystal holder and work through contact with the shield. Aninductance coi180 is connected between line 50 and condenser 28 and hasin parallel denser 62 for tuning the output circuit of the oscillator.In this case, also, a small compensating condenser 64 is connected inparallel with the main tuning condenser 62 and has in series therewith asmall loading resistor 66 whose opposite terminal is connected back toline 50. An inductance coil 68 is connected to the lower extremity ofthe tuned circuit consisting of inductor 60 and tuning condenser 62 andhas its opposite terminal connected to a current indicating device or amilliameter 10.

Line 40, ther output of the voltage supply filter, is also connected tolimiting resistor 12, which is in turn connected to the oppositeterminal of the milliameter 10. A condenser 14 is connected between themeter and ground. This point is also connected through a variableresistance 16 and a limiting resistance 18 to the output of a rectiilerbridge 80, the opposite terminal of which is connected through line 82back to the milliameter 10. A condenser 84 is connected across lines 82and 86. The inputto the rectifier bridge is through lines 88 and 80which are fed from a small secondary coil 92 on a small transformer. Theprimary coils 96 of this and 94 of the main transformer are fed bysuitable 110 volt input through lines 98 and |00. A second secondarycoil |02 of the main transformer has its center tap grounded throughline |04 andhas its two outside terminals connected through line |06 andline |08 to electrodes ||0 and ||2, respectively, of a full-waverectifier tube ||4, electrode ||6 of which is connected through line |I8to the filter system 42, 44, 46, previously described. It will be notedalso that the input and output tuned circuits for the oscillator arecommonly actuated as shown by the dot-and-dash line connecting the twoand are designed to track while being used to vary the oscillatorfrequency. The small compensating condenser 64, which is ganged with thetuning control, is adjusted so that the oscillator plate current Withoutexternal load is nearly constant over its frequency range.

All of the electrical system is, of course, enclosed within a casingindicated generally at |20 in Figure 1. The milliameter 10 is shown asmounted on the front panel and a common dial |24 is also shown thereonwhich controls the tuning of the two condensers I and 62. The line 54 ofFigure 3 is enclosed within a protected flexible cable |26 and leads tothe holder 56 which supports the crystal. This holder, best shown inFigure 2, comprises a long cylindrical metallic conductive casing |28grounded through'screws |52 to the cable shield |26 having at its lowerextremity three projections |30 for holding the base |32 up from thesurface of a part to be tested and having a long central opening |34throughout the main length of the housing which has an internal flange|36 near the lower end of said central opening.

The crystal 58 is rigidly secured to a circular disc |38 of insulatingmaterial such as Bakelite which has an enlarged external portion |40adapted to engage the flange |36 and prevent the passage of this memberentirely through the opening. Disc |38 also has a central opening |42therein which is adapted to provide space for the coiled end |44 of theconductor which contacts an electrode on the upper surface of thecrystal and acts as a spring to tend to eject the Bakelite member |38from the opening and at the same time conduct current from theintherewith a tuning concoming line 54 to the crystal. The conductor 04for the high frequency currents is supported within a cable formed ofshielding outer conductor |46 within which are a` series of insulatedbeads |48 to maintain the conductor 54 within the central portionthereof. A second tubular insulator |50 supports the end of the cable|26 and is itself maintained within the outer housing |28 by suitablegrounding set screws |52.

Crystals are most sensitive to thickness vibrations near or somewhatlower in frequency than their own thickness resonant frequency. Sincethe crystals own resonant frequency may introduce confusion, it ischosen outside the oscillator range being used and it is desirable tochange crystals when changing ranges. It is then only necessary towithdraw the set screws, take out the inner section including the member|50, allow the Bakelite disc holding the crystal 58 to be withdrawn fromthe upper portion of the holder and insert another Bakelite disc havingthe crystal of proper characteristics. e When this holder is applied tothe upper surface of a sheet of metal, such as |54, whose thickness itis desired to ascertain, through the three point suspension orprojections |30 it will accommodate itself to almost any surface, thecrystal is forced down into as fiat contact as possible with the uppersurface of the sheet |54 by the resiliency of the coiled portion |44.This places a certain load thereon and is sufficient to causesatisfactory contact between the two. In order to obtain good coupling,the crystal is first dipped in oil which then forms a film between thecrystal and the work, aiding in the transfer of motion at all points.

In operation, therefore, the device is actuated by the fact that theelectrical characteristics of certain mechanically loaded members changewith the mechanical loading. This might be the impedance of a coil whichwould change as the stress in an associated part of its magnetic circuitwas varied, or an impedance change in a condenser due to a change inmechanical stress or deformation of the dielectric, any of these changesbeing induced by a factor depending on the frequency or frequencies andamplitude of resonance of the measured part. If a wave whose length inthe material being measured is equal` to twice the thickness of themember being measured is applied by means of a transducer to the member,the same will vibrate in resonance. The transducer is thereforemechanically coupled to the vibrating member, the reacting forcesbetween the two will change and this change will affect the electricalcharacteristics of the transducer and may be used to indicate thisresonance point. In the present instance a piezoelectric crystal isshown, but this is only illustrative and any one Iof a number of otherdevices having the characteristics above pointed out may be used withinthe scope of my invention.

The operator rst dips the crystal in oil or other suitable liquid andapplies the holder to the surface. The crystal contacts the surface andisl pressed thereagainst to the desired degree. Now by applyingalternating voltage to the crystal, the same will be forced to vibrateat the frequency of the voltage and this vibration will be mechanicallycoupled through the oil film to the member being tested, forcing '1t tovibrate. If the proper frequency is applied to this sheet |54 so thatthe thickness thereof is equal to one-half wave length or integralmultiples thereof, then resonance can be set up in this sheet and thebody will vibrate freely with relatively large mechanical amplitudecompared to theolfresonance or forced vibrations. The operator thereforeslowly changes the frequency of the applied voltage by turning theoscillator dial |24 slowly through the range to try to impress thisfrequency on the'crystal and work. When this work resonance point isreached, the mechanical power loading on the quartz crystal 56 will besubstantially increased and its electrical characteristics changed sothat the power output of the circuit as measured by the plate current issharply increased.

If, therefore, there is provided in this circuit some means for closelyobserving the power output or loading and this resonance point checkedas the frequency of the oscillator is varied, then by knowing thefrequency at this point the thickness of the material can be found. Theresonance formula for steel for ascertaining the thickness is:

f: 128t0c0 where f is the frequency in cycles per second, and t is thethickness of the sheet in inches.

Through this formula we may calibrate the dial |56 for steel thickness,or by similar formula for any other material (solid, liquid or gas) and/or mode at resonance, rather than oscillator frequency, or the dial maybe calibrated empirically by locating certain points through using knownthicknesses of material for the same.

With the above description in mind, it will be seen that if thereare-any variations in the noload plate current as the tuner is swungover the band, these variations would tend to give false indications tothe operator when the crystal was in test position. It is therefore veryessential to use such care in designing the oscillator circuit thatthere will be as little variation as possible. In order to overcome thisvariation deficiency, I have provided a compensating condenser shown at64 in Figure 3 which is in reality a loaded plate attached to thecondenser and by adjusting the loading the no-load plate current is madeinitially constant throughout the band.

It may also be desirable to balance to zero or some other index pointthe current owing when the device has been applied to a sheet beforetuning begins and in order to so set the milliameter to such a desiredfigure, a biasing or opposing current is supplied which may be adjustedby a small rheostat 16 within the casing |20. This current is suppliedfrom the main source through the bridge rectier 80. When now theoperator holding the crystal in close contact with the sheet and theresistor 16 so set that the milliameter points to zero, the dial isswung over the range slowly and the point at which the milliameter givesa sudden motion upward indicating a peak current is that of thefrequency of resonance of the member. Then by either applying theformula or reading directly from the dial, the exact thickness of themember can be determined.

In some instances it may be advisable to utilize other connections fortheV milliameter instead of those shown in Figure 3 to simplify thesystem and Figures 4 and 5 show two such modified forms of connections.In Figure 4 the output of the supply filter 42, 44, 46 is applied to oneend of a bleed resistor |58, the opposite end of which is grounded, themilliameter 10 in this instance being connected to an intermediate pointof this resistance through line |60, the opposite terminal beingconnected through line |62 to a further resistor |64 in series with thecoil 68 of Figure 3. A variable resistance |66 or rheostat is connectedbetween line |62 and the filter. It will be seen that these elementsform a bridge circuit in which the input is across the two parts of theresistor |58 and the meter is across the bridge. Thus by adjusting thisrheostat, the bridge may be balanced andv the milliameter adjusted toits zero or indexed position.

A further modification is shown in Figure 5 in which the output from thefull-wave rectifier ||4 is connected through filter 48, 42, 44, to line|68 and thence to one terminal of the milliameter 10. The oppositeterminal of the milliameter 10 is connected through line |10 to theYplate |12 of an ampller tube, the grid |14 of which is connected throughline |16 to the coil 68 of Figure 3. A resistor 18 is connected betweenthis coil, |16 and the milliameter. The cathode |80 of the amplier tubeor triode is connected through line |82 to a variable tap |84 on aresistor |66 connected between line |68 and ground. Here, again, abridge circuit is formed, the resistors |18 and |86 forming arms thereofand the oscillator tube plate resistance a further arm. The amplifiertube grid and cathode are connected to the bridge output and its platevoltage obtained from the power supply by connection to the top of thebridge. The milliameter is connected in the amplier tube plate circuitand the bridge is balanced by movement of the tap |84 on resistor |86 togive the proper index. Any unbalance caused by a variation in thevoltage upon the grid |14 will cause the plate current to vary Vand areading will be obtained on the meter 10.

In all these instances any variation in the voltage supply iscompensated for inasmuch as the biasing voltage is changed in the samedegree as the voltage to the oscillator and therefore does not aiect thereadings.

In the above discussion, the work has been described as a homogeneoussheet in which the standing wave is set up by the transducer. If twopieces have been bonded together, they a pear to the measuring means asa single part if the bond is satisfactory. However, if the bond.

at a particular point is not good and a small spacing between the piecesoccurs, then the standlng wave is set up in only that part between thetransducer and the void or opening and the reading will be of thisdistance. Thus a change from a thickness reading of the composite bodyto a fractional distance will indicate at once a poor bond or fault inthe work.

So far the discussion has dealt only with the location of the resonancepoint, but there is utility in measuring the amplitude of the output atthe resonance point as Well. If the tuning dial is turned to a, giventhickness and the transducer brought into contact with a series ofsimilar parts in succession which are all presumed to be of the samethickness, then a variation in the amplitude of the resonance peak asindicated on the meter may indicate a slight variation in the characterof the material and limits may be set up to cause rejection of partsthrough this inspection.

These are only a few of the many uses in which the above system may beemployed and are therefore to be considered only as illustrative ofapplicants broad invention. It will therefore be obvious that I haveprovided a system capable of measuring,l sorting or studying parts byusing supersonic vibrations.

I claim:

1. In measuring means, an oscillator tunable over a predetermined band,a vibratable crystal connected to the oscillator output and capable ofvibrating to the frequencies generated by the oscillator, a source ofpower, D. C. current indicating means to measure the oscillator plateinput and means interconnecting said source and said indicating meanssothat fluctuations in supply line voltage will be compensated for, saidinterconnecting means being adjustable to balance the indicating meansfor initial setting.

2. In measuring means for indicating characteristics of a part, avibratable crystal capable of vibrating over a predetermined range, aholder, means for insulatably movably mounting the crystal within theholder to firmly contact the part and spring biasing means tending toeject the crystal from the holder.

3. In measuring means for indicating characteristics of a part, avibratable crystal capable of vibrating over a predetermined range, aholder, means for insulatably movably mounting the crystal within theholder, spring biasing means tending to eject the crystal from theholder to firmly contact the part, current conducting means supportedwithin the holder, said biasing means being connected thereto to serveboth to conduct the current to the crystal and to spring bias thelatter.-

4. In measuring means, a vibratable crystal capable of vibrating over apredetermined range, a holder, means for insulatably movably mountingthe crystal within the holder, spring biasing means tending to eject thecrystal from the holder, current conducting means supported within theholder, said biasing means being connected thereto to serve bothto-conduct the current to the crystal and to spring bias the latter, andremovable means for retaining the current conducting means in the holderso that it may be removed and crystals exchanged.

5. In measuring means, an oscillator tunable over a predeterminedsupersonic frequency band, a source of D. C. power, direct currentindicating means connected between the source and the oscillator, avibratable crystal to be pressed against a part to be measured connectedto the oscillator output so that it will impress vibrations upon thepart and set the same in motion and as the oscillator is tuned over aband the loading on the crystal will vary at resonance of the part whichvariation will be indicated upon the indicating means.

6..In measuring means, an oscillator tunable over a predetermined band,a vibratable crystal connected to the oscillator output and capable ofvibrating at the frequencies generated by the oscillator, a source ofpower, D. C. current indicating means connected between the source andthe oscillator, a rectifier bridge circuit separately connected to thesource and the current indicating means to initially set the indicatingmeans and to balance out the effect of fluctuations in voltage of thesource.

7. In measuring means, an oscillator for producing power in an outputcircuit over a p'redetermined range of frequency, a direct current inputcircuit connected to said oscillator, current indicating means in saidinput circuit, variable capacity tuning means in said oscillatorcircuit, auxiliary capacity means, resistance means in series relationthereto, said auxiliary capacity and re- 8 sistance being connected inparallel with the tuning capacity, common means for varying bothcapacity means as the oscillator is tuned over the band so that avariable compensatingload is introduced into the circuit so as tomaintain the current indication in the input circuit substantiallyconstant at no-load.

8. In a bridge type measuring circuit, a plurality of resistors, one ofwhich is variable, which form a majority of the arms of said bridge, anoscillator tunable over a predetermined band, a vibratable crystalconnected in the oscillator output and capable of vibrating atfrequencies generated by the oscillator, said oscillator output circuitforming the remaining arm of said bridge, a D. C. current indicatingmeans diagonally connected across the bridge, a D. C. source of powerconnected across the opposite corners of said bridge so that saidindicating means is responsive to changes in impedance of the oscillatoroutput circuit but is relatively unresponsive to changes in voltage ofsaid source. said variable resistance arm of said bridge providing meansfor setting the current indicating means to a predetermined index.

9. In measuring means for indicating characteristics of a part, avibratable crystal, insulating mounting means supporting the crystal atits perimeter, a holder, spring biasing means tending to eject thecrystal and mounting from the holder so that the crystal may firmlyengage a part to betested.

10. In measuring means, a supersonic oscillator, tuning means for saidoscillator, a vibratable crystal connected to said oscillator output, asource of D. C. power, an ammeter connected t0 the source and to theoscillator to indicate current ow thereto, a dial on said tuning meansso that when the crystal is coupled to a part to be tested and theoscillator tuned over its band the variation in load on said crystal atthickness resonance of the part will be indicated by the meter and thereading as shown on the dial will give the thickness of the part.

l1. In measuring means, an oscillator for producing power in an outputcircuit over s, predetermined range of frequency, a direct current inputcircuit connected to said oscillator, current indicating means in saidinput circuit, variable tuning means in said oscillator circuit,auxiliary means connected in said oscillator circuit for variablyresistance loading the oscillator circuit, mechanical means connectingthe auxiliary means with the tuning means to vary the resistance loadingas a predetermined function of the frequency tuning means so that at anyfrequency set by the tuning means the oscillator will be loaded tosubstantially the same direct current input current.

WESLEY S. ERWIN.

REFERENCES CITED The following references are ofl record in the file ofthis patent:

(Other references on following page) Y"Certificate of Correction 9Number Name Date 1,450,246 Cady Apr. 3, 1923 1,465,352 Dobson Aug. 21,1923 1,880,425 Flanders Oct, 4, 1932 2,178,252 Forster Oct. 31, 19392,188,830 Clark et al. Jan. 30, 1940 2,198,226 Peterson Apr. 23, 19402,329,321 Bach Sept. 14, 1943 1,844,705 Trip Feb. 9, 1932 Patent No.2,431,233.

It is hereby certified that error a numbered patent requiring correctionas follows: Column 7,

WESLEY S. ERWIN ppeers in the Number November 18, 1947.

printed specification of the above line 19, claim 2, after the wordholder strike out to firmly contact the part and insert the same afterholder and before the perio read with this correction th case in thePatent Oce.

Signed and sealed this 20th day of January, A. D. 1948.

d in line 21; and that the said Letters Patent should be erein that theseme may conform to the record of the THOMAS F. MURPHY,

Assistant ommz'asoner of Patents.

